Aerial service wire which has been referred to as drop wire extends the communications loop from distribution cable terminals to customers' premises. Typically, telephone aerial service wire comprises two parallel spaced metallic conductors. The metallic conductors not only provide transmission paths, but also they provide strength for aerial installations.
In one prior art aerial service wire, it was customary to include two electroformed copper-clad steel conductors enclosed with an extruded rubber insulation covered by a textile serving, and jacketed with a rubber-like material. Although such protective coverings were in widespread use for many years and had proven satisfactory from most standpoints, there developed a long felt desire for an alternative, less expensive covering. Any alternative covering had to have the capability of withstanding exposure to the elements, as well as adequate low temperature flexibility, impact resistance, and abrasion resistance.
A substantially improved plastic covered aerial service wire is disclosed in U.S. Pat. No. 3,935,369 issued on Jan. 27, 1976 to E. J. George, et al and includes two copper-clad steel conductors enclosed in insulation comprising a plasticized polyvinyl chloride (PVC) composition. The insulation serves also as a jacket. In addition to being more economical, this last described aerial service wire is lighter, more abrasion-resistant and easier to handle and terminate than its predecessor.
In a typical installation, each end portion of the aerial service wire is inserted into a metallic clamp. One of the clamps is attached to a customer's premises and the other one to a pole prior to electrical connections being made to wiring inside the customer's premises and to aerial distribution cables, respectively. At both the customer and the pole end of the aerial service wire, the wire is retained in engagement with the clamp by reactive forces exerted by the clamp on the insulation. If these forces are not transferred from the insulation to the conductors by coupling therebetween, the conductors will slip relative to the insulation within the clamp causing the insulation to rupture and, in some instances, the wire to fall. Adhesion of the plastic material to the metallic conductors may provide suitable coupling.
On the other hand, if the adhesion between the insulation and the conductors is too great, stripping the covering from the conductors may be difficult. Excessive scraping causes some of the copper from the surface portion of the copper-clad steel conductor to be removed thereby increasing the electrical resistance of the conductor termination. Excessive scraping also reduces the cross-sectional area of the conductor or nicks it thereby reducing its strength properties to the detriment of its load-supporting capability. Also, scraping may expose the steel, promoting corrosion of the bimetallic conductor.
Although present aerial service wires are adequate, a new, less expensive design has been sought. Environmental requirements for the disposal of residual copper and processing chemicals have raised the cost of manufacturing copper-clad conductors. Also, these conductors, on occasion, have displayed another problem. The problem is the result of the smooth surface of the drawn metallic conductor which does not adhere to the PVC jacket as well as the rough surface of the earlier manufactured electroformed conductor. This problem has been solved by applying electrostatically an adhesive to preheated conductors prior to the extrusion of the jacket material, but this solution involves an additional step.
The presently used aerial service wire presents some other problems. It is relatively stiff and somewhat difficult to handle. Stripping for termination requires separation of the insulated conductors with diagonal pliers, crushing of the insulation in rear jaws of the pliers, and removal of the insulation and adhesive residues by a stripper notch of the pliers. Also, the wire cannot be terminated in insulation displacement type connectors because of the relatively thick PVC insulation which toughens as temperature decreases and because copper-steel conductors are not easily and reliably accommodated by these kinds of connectors. Further cost reductions of the design are difficult to achieve inasmuch as the PVC material serves as both the jacket and conductor insulation, thereby requiring a balancing of mechanical and electrical requirements.
A most important reason for seeking a new design is the corrosion of the bimetallic conductor which occurs in coastal and industrial areas. Corrosion appears where a conductor has been stripped for termination, where a conductor has been accidentally nicked during separation of the conductors by pliers, or where maintenance tests have been made improperly by inserting test pins through insulation to a conductor.
What is needed and what is not provided by the prior art is an aerial service transmission medium which provides adequate protection against corrosion and which at least meets the strength properties of the prior art copper-clad steel design. The strength members of the sought-after aerial service transmission medium must have suitable coupling to the jacket in order to make it compatible with present support schemes at customers' premises. Further, it is a desire to have the conductors insulated with a material other than that used for the jacket in order to access the conductors without causing damage and to separate the electrical requirements of insulation from the mechanical requirements of the jacket. Still further, the design should be one which easily may be adaptable to optical fiber loop transmission.